1. Field of the Invention:
The present invention relates to plasma sources. More specifically, the present invention relates to low energy plasma sources.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art:
Conventional plasma sources can be broadly grouped on the basis of the mode in which a discharge of electrons into a gas is effected to form a plasma. In particular, plasma sources characterized by a dc (Penning) electron discharge typically include an element for thermionically emitting a supply of ionizing electrons. Alternatively, other plasma sources effect electron discharge through application of relatively high-voltage radio frequency (rf) electric fields to the gas.
Penning-type sources generally include either a filamentary cathode or a "hollow" cathode for achieving thermionic electron emission. Hollow cathodes include a chamber containing an insert having a coating of electron emissive material. The emissive material, when heated, furnishes the desired supply of ionizing electrons. Similarly, thermionic electron emission may also be achieved by passing a current of sufficient magnitude through a filamentary wire. Following electron emission a plasma is formed in Penning-type sources by accelerating the emitted electrons through the gas by way of a suitably biased anode. Ionization efficiency is enhanced by gyrating the path of the accelerated electrons with a magnetic field.
However, plasma sources operative through Penning discharges facilitated by electrons from filamentary cathodes are unsuitable for certain long term applications. Specifically, with prolonged usage such filamentary cathodes predictably burn out. Consequently such filamentary cathodes are unacceptable for utilization in, for example, spacecraft applications wherein replacement of the cathode is not possible.
Similarly, hollow cathodes used in Penning-type sources are not implementable in many plasma sources fueled by reactive gasses. Reactive gasses such as oxygen are desirable plasma fuels in applications such as plasma cleaning of optical surfaces. For example, optical surfaces included within a spacecraft may become contaminated with molecular films due to the outgassing of organic substances in the vicinity of the optical elements. These elements may be cleaned by allowing the plasma generated from the reactive gas to react with contaminants present on the optical surface. Under appropriate conditions the energy of reaction between these contaminants and the cleaning plasma is sufficient such that reaction products containing the contaminants are liberated from the optical surface. However, the electron emissive material which coats the hollow cathode is often incompatible with such reactive gasses. Hence, conventional plasma sources employing hollow cathodes and Penning discharges are not suitable for cleaning applications involving reactive gasses.
As mentioned above, other plasma sources effect plasma formation by ionizing a gas through application of a sufficiently large rf electric field. These "rf-only" sources may be fed by reactive gas mixtures and do not require a supply of thermionically generated electrons. However, rf-only sources are typically inefficient in comparison with Penning-type sources. That is, rf-only sources dissipate more energy per ion produced than do Penning-type sources. This inefficiency stems in part from the high voltages necessary for adequate ionization of a gaseous fuel and makes rf-only sources less attractive in spacecraft applications where available power is limited. Further, these high rf voltages also produce sputtering of metal atoms within the discharge chamber in which ionization occurs. These sputtered metal atoms have a high probability of becoming ionized and consequently becoming plasma contaminants. In addition, plasmas generated by rf-only sources are typically of relatively high energy. High energy plasmas are unsuitable for use in applications such as optical cleaning as the energetic ions included therein tend to roughen sensitive optical surfaces thereby degrading performance.
Hence, a need in the art exists for a low energy, long lifetime plasma source which includes a cathode capable of utilizing reactive gases.